Past hurts that co-arise on account of the words or actions of others leave traces within our consciousness. Despite being behind us, even if we feel that we're "over it", when given the perfect set of conditions, the pain might rise again. The suffering that follows could be likened to an uninvited guest; the longer we entertain it, the longer it will feel inclined to stay.
When we have dialogues with ourselves, whether silently or aloud, retracing the steps of why we feel justified in our anger, we're in essence asking the suffering to stay as long as it likes. It's as if we've asked it to stay for dinner, fed it, and invited it to post up indefinitely in the spare bedroom! Enticing indeed, but not the best company to keep.
Being that the original circumstances that gave rise to the initial pain are in the past, the pain we're experiencing in this moment exists entirely within ourselves, and will only cease when we cease to feed it. Knowing that anger hurts us first, we might ask the question "why do I keep retelling a story that causes me pain?"
Through the regular practice of meditation, as we familiarize ourselves with the nature of the mind, we naturally come to see this knack of storytelling as a choice. We're choosing to mentally relive past experiences over and over again, and by doing so, we never allow forgiveness to come.
In the midst of this painful storytelling we might witness a critical moment in which we realize that our mind, holding onto the pain, conspires to retaliate by wanting the "perpetrator" to also feel pain. This directly and totally conflicts with the practice of loving kindness, through which we wish ourselves first, then all others, to be well and happy.
The way out is to be aware, to be vigilant, to notice when the mind descends into these stories and the ill feelings that accompany them, mentally noting "I'm doing it again." Then, knowing that we're the author, through mindfulness and loving kindness, we can abandon thoughts of ill will, cultivate thoughts of goodwill, and intentionally go about the work of the next chapter, writing toward a happy ending.